An electronic ballast is configured to provide a lamp current for energizing a set of lamps. A ballast includes an inverter circuit for generating an oscillating power signal, and the lamp current is produced from the oscillating power signal. Some ballasts are configured to provide a lamp current for energizing a set of lamps that includes a variable number of lamps. For example, a ballast may be configured to provide a lamp current for energizing up to two lamps that are connected together in series. Thus, the ballast energizes two lamps when both lamps are connected to the ballast, and energizes only one lamp when one of the lamps becomes disconnected from the ballast.
In general, such ballasts operate the inverter circuit at the same frequency (i.e., the oscillating power signal has the same frequency) even when the number of lamps has changed. Thus, when one of the two lamps is disconnected from the ballast, the lamp current provided to the remaining lamp increases beyond a nominal value.